474 FURTHER EVOLUTION 



followed parallel paths in different organisms even at that 

 stage of the development of the living world when there first 

 occurred a clear-cut differentiation into its main divisions. 



One may arrive at a similar conclusion from a study of the 

 numerous enzymes which take part in the chain of oxidative 

 transformations in different organisms. This specially con- 

 cerns the ' terminal ' group of catalysts which directly activate 

 molecular oxygen. In organisms which are far removed from 

 one another systematically this task is often accomplished by 

 widely different catalytic mechanisms. The earliest of these 

 would seem to be the cytochrome complex.^"" This, clearly, 

 owes its origin to the iron-porphyrin compounds of the 

 primaeval living things. Thus, as we have seen, cytochromes 

 are to be found in rather primitive anaerobic organisms. 



With the appearance of molecular oxygen in the atmo- 

 sphere the most diverse representatives of the living world 

 could easily make use of the cytochromes present in them 

 as oxidase mechanisms, adapting them to the activation of 

 oxygen in the process of respiration. 



As a result of this the cytochromes and the corresponding 

 enzymes, the cytochrome oxidases, seem to be very widely 

 distributed respiratory mechanisms ; we find them in a wide 

 variety of systematic groups of organisms, but their import- 

 ance is especially great in the respiratory processes of a 

 number of micro-organisms as well as in animal cells. In 

 higher plants the most important part in this connection 

 falls to the phenol oxidase system in which the enzymes are 

 copper-proteins,^" and the transporters of hydrogen are the 

 * respiratory chromogens ' of Palladin, especially chlorogenic 

 acid.^°^ These mechanisms are highly specific to plants and 

 are completely absent from members of the animal kingdom. 

 It is evident that in the process of phylogenesis they were 

 elaborated after the separation of organisms into the animal 

 and vegetable kingdoms. 



In the ontogenesis of a number of plants we may also 

 observe that the cytochrome oxidase mechanism only plays 

 a leading part during the embryonic stage of development 

 when the plant is still leading a heterotrophic life.^"^ How- 

 ever, with the emergence of autotrophy in the plant we can 



